U.S. patent number 10,492,161 [Application Number 15/571,450] was granted by the patent office on 2019-11-26 for method and device for acquiring uplink synchronism in consideration of beam forming effect in wireless communication system.
This patent grant is currently assigned to LG ELECTRONICS INC.. The grantee listed for this patent is LG ELECTRONICS INC.. Invention is credited to Jiwon Kang, Heejin Kim, Kitae Kim, Kilbom Lee, Kungmin Park.
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United States Patent |
10,492,161 |
Kim , et al. |
November 26, 2019 |
Method and device for acquiring uplink synchronism in consideration
of beam forming effect in wireless communication system
Abstract
Provided is a method for estimating a timing advance (TA) for
each beam in a wireless communication system. First, a terminal
transmits K preambles, to which K mutually different beam formings
are applied, to a base station. The base station estimates the TA
for each of the K received preambles. The base station can
determine the final TA value of the terminal on the basis of the
TAs estimated for each of the K preambles.
Inventors: |
Kim; Kitae (Seoul,
KR), Kang; Jiwon (Seoul, KR), Lee;
Kilbom (Seoul, KR), Park; Kungmin (Seoul,
KR), Kim; Heejin (Seoul, KR) |
Applicant: |
Name |
City |
State |
Country |
Type |
LG ELECTRONICS INC. |
Seoul |
N/A |
KR |
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Assignee: |
LG ELECTRONICS INC. (Seoul,
KR)
|
Family
ID: |
57218598 |
Appl.
No.: |
15/571,450 |
Filed: |
May 9, 2016 |
PCT
Filed: |
May 09, 2016 |
PCT No.: |
PCT/KR2016/004791 |
371(c)(1),(2),(4) Date: |
November 02, 2017 |
PCT
Pub. No.: |
WO2016/178546 |
PCT
Pub. Date: |
November 10, 2016 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20190159154 A1 |
May 23, 2019 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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62157478 |
May 6, 2015 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04W
72/046 (20130101); H04W 56/0045 (20130101); H04B
7/0617 (20130101); H04W 56/001 (20130101); H04W
72/04 (20130101); H04B 7/06 (20130101); H04B
7/0456 (20130101); H04L 5/0007 (20130101) |
Current International
Class: |
H04W
56/00 (20090101); H04W 72/04 (20090101); H04B
7/06 (20060101); H04B 7/0456 (20170101); H04L
5/00 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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10-2014-0002558 |
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Jan 2014 |
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KR |
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WO 2013/095003 |
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Jun 2013 |
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WO |
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WO 2014/109569 |
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Jul 2014 |
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WO |
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WO 2015/030524 |
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Mar 2015 |
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WO |
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Primary Examiner: Gidado; Rasheed
Attorney, Agent or Firm: Birch, Stewart, Kolasch &
Birch, LLP
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a National Phase of PCT International
Application No. PCT/KR2016/004791, filed on May 9, 2016, which
claims priority under 35U.S.C. 119(e) to U.S. Provisional
Application No. 62/157,478, filed on May 6, 2015, all of which are
hereby expressly incorporated by reference into the present
application.
Claims
What is claimed is:
1. A method for estimating, by a base station (BS), a timing
advance (TA) for each beam in a wireless communication system, the
method comprising: receiving K preambles, to which K different
beamformings are applied, from a user equipment (UE), where K is a
number; estimating a channel delay for each of the K preambles
based on the following equation:
.times..times..times..times..times..times..times..function..times..functi-
on. ##EQU00005## where i is a time index, m is a timing offset, N
is a total length of a time signal (or orthogonal frequency
division multiplexing (OFDM) symbol length), L is a multipath
channel delay length by probability distribution function (PDF), Y
is a signal received at time i, and S is a signal transmitted at
time i; and estimating the TA for the each of the K preambles based
on the estimated channel delay for the each of the K preambles.
2. The method of claim 1, wherein the K preambles are received
through different subframes.
3. The method of claim 1, wherein the K preambles are successively
received in a same subframe.
4. The method of claim 1, further comprising determining a final TA
value of the UE based on the estimated TA for the each of the K
preambles.
5. The method of claim 4, wherein the final TA value of the UE is
determined to be a TA value related to a preamble having a best
reception quality among the K preambles.
6. The method of claim 4, wherein the final TA value of the UE is
determined to be an average value of a TA value used for
conventional synchronization and a TA value related to a preamble
having a best reception quality among the K preambles.
7. The method of claim 4, wherein the final TA value of the UE is
determined based on TA values related to two or more preambles
among the K preambles.
8. The method of claim 7, wherein the final TA value of the UE is
determined to be an average value of a TA value related to a
preamble having a best reception quality among the K preambles and
a TA value related to a preamble having a second best reception
quality.
9. The method of claim 7, wherein the final TA value of the UE is
determined to be an average value of TA values related to preambles
having a reception quality of a specified threshold or higher among
the K preambles.
10. The method of claim 4, further comprising transmitting a TA
command comprising the determined final TA value of the UE to the
UE.
11. The method of claim 1, wherein the beamformings comprise at
least one of analog beamforming, digital beamforming, or hybrid
beamforming.
12. A base station (BS) for estimating a timing advance (TA) for
each beam, the BS comprising: a transceiver; and a processor
operatively coupled to the transceiver, wherein the processor is
configured to: control the transceiver to receive K preambles, to
which K different beamformings are applied, from a user equipment
(UE), where K is a number; estimate a channel delay for each of the
K preambles based on the following equation:
.times..times..times..times..times..times..times..function..times..functi-
on. ##EQU00006## where i is a time index, m is a timing offset, N
is a total length of a time signal (or orthogonal frequency
division multiplexing (OFDM) symbol length), L is a multipath
channel delay length by probability distribution function (PDF), Y
is a signal received at time i, and S is a signal transmitted at
time i; and estimate the TA for the each of the K preambles based
on the estimated channel delay for the each of the K preambles.
13. The BS of claim 12, wherein the processor is further configured
to: determine a final TA value of the UE based on the estimated TA
for the each of the K preambles.
14. The BS of claim 13, wherein the final TA value of the UE is
determined to be a TA value related to a preamble having a best
reception quality among the K preambles.
15. The BS of claim 13, wherein the final TA value of the UE is
determined based on TA values related to two or more preambles
among the K preambles.
16. The BS of claim 15, wherein the final TA value of the UE is
determined to be an average value of TA values related to preambles
having a reception quality of a specified threshold or higher among
the K preambles.
17. The BS of claim 13, wherein the processor is further configured
to: control the transceiver to transmit a TA command comprising the
determined final TA value of the UE to the UE.
Description
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to wireless communication, and more
particularly, to a method and a device for acquiring uplink
synchronization in consideration of the effect of beamforming in a
wireless communication system.
Related Art
3rd generation partnership project (3GPP) long-term evolution (LTE)
is a technology for enabling high-speed packet communications. Many
schemes have been proposed for the LTE objective including those
that aim to reduce user and provider costs, improve service
quality, and expand and improve coverage and system capacity. The
3GPP LTE requires reduced cost per bit, increased service
availability, flexible use of a frequency band, a simple structure,
an open interface, and adequate power consumption of a terminal as
an upper-level requirement.
As a way to improve the efficiency of limited resources, so-called
multi-antenna technology is being actively developed which achieves
diversity gain by putting multiple antennas on the transmitter and
receiver to cover more spatial regions for resource utilization,
and which increases transmission capacity by transmitting data in
parallel through each antenna. Multi-antenna technology may employ
beamforming and/or precoding to improve signal-to-noise ratio
(SNR). In closed-loop systems that can use feedback information at
the transmitting end, beamforming and/or precoding may be used to
maximize SNR through such feedback information. Beamforming is
broadly classified into analog beamforming and digital
beamforming.
Massive multiple-input multiple-output (MIMO) is a multi-antenna
technology in which tens of antennas or even more, which is a lot
more than now, are put into a base station to achieve higher data
rates and higher energy efficiency. When conventional analog
beamforming and/or digital beamforming is directly used in massive
MIMO, signal processing and/or hardware implementation can get very
complex, or the performance increase through the use of multiple
antennas is only slight and the flexibility of resource allocation
may be reduced. Consequently, the use of hybrid beamforming, a
combination of conventional analog and digital beamforming, in
massive MIMO is under discussion.
Recently, due to the rapid spread of mobile smart devices and the
emergence of big data, mobile traffic is expected to be doubled
every year and increased more than 1000 times in 10 years. The
burden of mobile network operators have been increased due to the
explosion of mobile traffic, and existing 4G mobile communication
systems with limited additional frequency coverage cannot
accommodate the explosive mobile traffic. Therefore, the
development of 5th generation mobile communication technology based
on millimeter wave (mmWave) capable of securing broadband is being
discussed. The millimeter wave is a frequency band of 30-300 GHz
which is generally called extremely high frequency (EHF) band and
has its wavelength of 1 cm to 1 mm. The wave with the wavelength is
in the middle of the currently used radio frequency band and the
infrared ray (its wavelength about 0.1 mm), and it is very close to
the light and is used in high resolution radar and microwave
spectroscopy. The millimeter wave has less diffraction properties
and larger directive properties than the conventional communication
wave, and has larger diffraction properties, and less directive
properties than the laser beam. When millimeter waves are used for
communication, it is considered that ultra-multiple communications
is possible in that far exceeds the microwave communication
capacity, but there is large transmission loss in the spatial
transmission. This is because the energy absorption by the oxygen
and water molecules in the atmosphere is relatively large compared
to the existing cellular frequency, resulting in high path
loss.
When hybrid beamforming is introduced, individual beams may have
different channel characteristics and thus may have different
channel delays. Therefore, a method for acquiring uplink
synchronization different from a conventional method may be
required in consideration of such characteristics.
SUMMARY OF THE INVENTION
The present invention provides a method and a device for acquiring
uplink synchronization in consideration of the effect of
beamforming in a wireless communication system. The present
invention provides a method and a device for solving timing
misalignment per beam, which occurs due to change of channel
characteristic per beam when uplink beam scanning is performed. The
present invention also provides a method and a device for
estimating a channel delay per beam, and finally determining a
timing advance value
In an aspect, a method for estimating, by a base station (BS), a
timing advance (TA) for each beam in a wireless communication
system is provided. The method includes receiving K preambles, to
which K different beamformings are applied, from a user equipment
(UE), and estimating a TA for each of the K preambles.
In another aspect, a method for applying, by a user equipment (UE),
a timing advance (TA) in a wireless communication system is
provided. The method includes transmitting K preambles, to which K
different beamformings are applied, to a base station (BS), and
receiving a TA command comprising a final TA value, which is
determined by the BS based on the K preambles, from the BS.
It is possible to prevent timing misalignment between beams caused
by a change in channel characteristics for each beam.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a cellular system.
FIG. 2 shows a structure of a radio frame of 3GPP LTE.
FIG. 3 is a block diagram of a transmitter including an analog
beamformer and a radio frequency (RF) chain.
FIG. 4 is a block diagram of a transmitter including a digital
beamformer and a RF chain.
FIG. 5 is a block diagram of a transmitter including a hybrid
beamformer.
FIG. 6 shows an example of timing alignment in UL transmission to
which no TA is applied.
FIG. 7 shows an example of timing alignment in UL transmission to
which TA is applied.
FIG. 8 shows an example of applying a TA command for TA update.
FIG. 9 is a conceptual view showing that a channel characteristic
changes by beam.
FIG. 10 is a conceptual diagram showing that each beam has a
different channel characteristic.
FIG. 11 shows an example of a preamble transmitted by a UE for UL
beam scanning of a BS according to an embodiment of the present
invention.
FIG. 12 shows an example of different channel characteristics and
different total delays for respective beams according to an
embodiment of the present invention.
FIG. 13 shows an example of determining a final TA value based on a
TA value estimated for each beam according to an embodiment of the
present invention.
FIG. 14 shows a method for a BS to estimate a TA for each beam
according to an embodiment of the present invention.
FIG. 15 shows a method for a UE to apply a TA according to an
embodiment of the present invention.
FIG. 16 shows a TA procedure according to an embodiment of the
present invention.
FIG. 17 shows a wireless communication system to implement an
embodiment of the present invention.
DESCRIPTION OF EXEMPLARY EMBODIMENTS
The technology described below can be used in various wireless
communication systems such as code division multiple access (CDMA),
frequency division multiple access (FDMA), time division multiple
access (TDMA), orthogonal frequency division multiple access
(OFDMA), single carrier frequency division multiple access
(SC-FDMA), etc. The CDMA can be implemented with a radio technology
such as universal terrestrial radio access (UTRA) or CDMA-2000. The
TDMA can be implemented with a radio technology such as global
system for mobile communications (GSM)/general packet ratio service
(GPRS)/enhanced data rate for GSM evolution (EDGE). The OFDMA can
be implemented with a radio technology such as institute of
electrical and electronics engineers (IEEE) 802.11 (Wi-Fi), IEEE
802.16 (WiMAX), IEEE 802-20, evolved UTRA (E-UTRA), etc. IEEE
802.16m is an evolution of IEEE 802.16e, and provides backward
compatibility with an IEEE 802.16-based system. The UTRA is a part
of a universal mobile telecommunication system (UMTS). 3rd
generation partnership project (3GPP) long term evolution (LTE) is
a part of an evolved UMTS (E-UMTS) using the E-UTRA. The 3GPP LTE
uses the OFDMA in downlink and uses the SC-FDMA in uplink.
LTE-advance (LTE-A) is an evolution of the 3GPP LTE.
For clarity, the following description will focus on the LTE-A.
However, technical features of the present invention are not
limited thereto.
FIG. 1 shows a cellular system. Referring to FIG. 1, the cellular
system 10 includes at least one base station (BS) 11. Respective
BSs 11 provide a communication service to particular geographical
areas 15a, 15b, and 15c (which are generally called cells). Each
cell may be divided into a plurality of areas (which are called
sectors). A user equipment (UE) 12 may be fixed or mobile and may
be referred to by other names such as mobile station (MS), mobile
terminal (MT), user terminal (UT), subscriber station (SS),
wireless device, personal digital assistant (PDA), wireless modem,
handheld device. The BS 11 generally refers to a fixed station that
communicates with the UE 12 and may be called by other names such
as evolved-NodeB (eNB), base transceiver system (BTS), access point
(AP), etc.
In general, a UE belongs to one cell, and the cell to which a UE
belongs is called a serving cell. A BS providing a communication
service to the serving cell is called a serving BS. The cellular
system includes a different cell adjacent to the serving cell. The
different cell adjacent to the serving cell is called a neighbor
cell. A BS providing a communication service to the neighbor cell
is called a neighbor BS. The serving cell and the neighbor cell are
relatively determined based on a UE.
This technique can be used for downlink (DL) or uplink (UL). In
general, DL refers to communication from the BS 11 to the UE 12,
and UL refers to communication from the UE 12 to the BS 11. In DL,
a transmitter may be part of the BS 11 and a receiver may be part
of the UE 12. In UL, a transmitter may be part of the UE 12 and a
receiver may be part of the BS 11.
FIG. 2 shows a structure of a radio frame of 3GPP LTE. Referring to
FIG. 2, a radio frame consists of 10 subframes, and a subframe
consists of two slots. Slots within the radio frame are numbered
from #0 to #19. A transmission time interval (TTI) is a basic
scheduling unit for data transmission. In 3GPP LTE, one TTI may be
equal to the time it takes for one subframe to be transmitted. One
radio frame may have a length of 10 ms, one subframe may have a
length of 1 ms, and one slot may have a length of 0.5 ms.
One slot includes a plurality of orthogonal frequency division
multiplexing (OFDM) symbols in time domain and a plurality of
subcarriers in frequency domain. Since the 3GPP LTE uses the OFDMA
in the DL, the OFDM symbol is for representing one symbol period.
The OFDM symbols may be called by other names depending on a
multiple-access scheme. For example, when SC-FDMA is in use as a UL
multi-access scheme, the OFDM symbols may be called SC-FDMA
symbols. A resource block (RB) is a resource allocation unit, and
includes a plurality of contiguous subcarriers in one slot. The
structure of the radio frame is shown for exemplary purposes only.
Thus, the number of subframes included in the radio frame or the
number of slots included in the subframe or the number of OFDM
symbols included in the slot may be modified in various manners.
3GPP LTE defines one slot as 7 OFDM symbols in a normal cyclic
prefix (CP) and one slot as 6 OFDM symbols in an extended CP.
The need for hybrid beamforming will be described. Beamforming
technology using multiple antennas may be broadly divided into
analog beamforming technology (hereinafter, "analog beamforming")
and digital beamforming technology (hereinafter, "digital
beamforming") depending on where a beamforming weight vector (or
precoding vector) is applied.
FIG. 3 is a block diagram of a transmitter including an analog
beamformer and a radio frequency (RF) chain. Analog beamforming is
a typical beamforming technique applied to earlier multi-antenna
structures. In analog beamforming, a beam is formed by partitioning
an analog signal, produced by digital signal processing, into a
plurality of paths and configuring the phase shift (PS) and power
amplifier (PA) settings for each path. Referring to FIG. 3, an
analog signal derived from a single digital signal is processed by
the PS and PA connected to each antenna. That is, the PS and the PA
handles complex weights in the analog stage. Here, the RF chain
refers to a processing block that converts a baseband signal into
an analog signal. In analog beamforming, beam precision is
determined by the element characteristics of the PS and PA, and the
control characteristics of the element make analog beamforming
advantageous for narrowband transmission. Moreover, the hardware
structure makes it difficult to implement multi-stream
transmission, thus making the multiplexing gain for higher data
rates relatively small and making it impractical to form a beam per
user based on orthogonal resource allocation.
FIG. 4 is a block diagram of a transmitter including a digital
beamformer and a RF chain. In digital beamforming, as opposed to
analog beamforming, a beam is formed in the digital stage by a
baseband process, in order to maximize diversity and multiplexing
gain in an MIMO environment. Referring to FIG. 4, a beam may be
formed by performing precoding in the baseband process. The RF
chains may include PAs. Hence, complex weights generated for
beamforming may be applied directly to transmitted data. Digital
beamforming may support simultaneous multi-user beamforming because
a different beam may be formed for each user. Moreover, digital
beamforming allows for forming an independent beam for each user to
whom a resource is orthogonally allocated, thus providing high
scheduling flexibility and enabling to operate the transmitting end
according to a system purpose. In digital beamforming, when a
technology such as MIMO-OFDM is used in a broadband transmission
environment, an independent beam may be formed per subcarrier.
Thus, digital beamforming may optimize the maximum data rate of a
single user on the basis of increased system capacity and greater
beam gain. Therefore, digital beamforming-based MIMO technology was
adopted to 3G/4G systems.
Meanwhile, a massive MIMO environment with a significantly
increased number of transmit-receive antennas may be considered.
For a typical cellular system, it is assumed that up to 8
transmit-receive antennas are used in an MIMO environment, whereas
scores or even hundreds of transmit-receive antennas may be used in
a massive MIMO environment. When conventional digital beamforming
is used in a massive MIMO environment, digital signal processing
should be performed for hundreds of transmit antennas by a baseband
process. This increases the complexity of signal processing
considerably, and also increases the complexity of hardware
implementation considerably since as many RF chains are needed as
there are transmit antennas. Moreover, independent channel
estimation is needed for every transmit antenna, and a frequency
division duplex (FDD) system requires feedback information for
massive MIMO channels of all antennas, thus considerably increasing
pilot and feedback overhead. In contrast, when conventional analog
beamforming is used in a massive MIMO environment, the hardware
complexity at the transmitting end is relatively low, but the
performance increase through the use of multiple antennas is only
slight and the flexibility of resource allocation may be reduced.
Especially in broadband transmission, it is very hard to perform
beam control for each frequency.
Accordingly, massive MIMO environments require hybrid beamforming,
a combination of analog beamforming and digital beamforming, rather
than using either analog beamforming or digital beamforming as a
beamforming technology. That is, a hybrid-type transmitting end
structure may be needed so as to lower the complexity of hardware
implementation at the transmitting end according to the
characteristics of analog beamforming and to maximize beamforming
gain using a large number of transmit antennas according to the
characteristics of digital beamforming.
Hybrid beamforming will be described. As described above, the
purpose of hybrid beamforming is to configure a transmitting end
that provides the benefits of analog beamforming and the benefits
of digital beamforming in a massive MIMO environment.
FIG. 5 is a block diagram of a transmitter including a hybrid
beamformer. Referring to FIG. 5, hybrid beamforming may basically
allow for forming a coarse beam through analog beamforming and then
a beam for multi-stream or multi-user transmission through digital
beamforming. That is, hybrid beamforming exploits both analog
beamforming and digital beamforming in order to lower the
complexity of implementation at the transmitting end or hardware
complexity.
Technical issues with hybrid beamforming are as follows.
(1) Difficulties in optimizing analog/digital beamforming designs:
While digital beamforming allows for forming individual beams for
different users using the same time-frequency resource, analog
beamforming is limited in that a common beam has to be formed using
the same time-frequency resource. This limitation may cause issues
like a limit on the largest possible number of ranks corresponding
to the number of RF chains, the difficulty of subband beam control
using an RF beamformer, and/or the difficulty of optimization of
beamforming resolution/granularity.
(2) Need for a specific method of common signal transmission: In
analog beamforming, which forms a beam only in a particular
direction on the same time-frequency resource, it is not possible
to form multiple beams simultaneously in the directions of all UEs.
Thus, DL/UL control channels, reference signals, broadcast
channels, synchronization signals, etc., may not be transmitted
simultaneously to all UEs that may be distributed over all areas in
a cell. There are also problems which occur when a UE transmits
physical random access channel (PRACH), physical uplink control
channel (PUCCH), and/or sounding RS (SRS) over UL.
(3) Need for the design of more pilots and feedback to determine an
analog/digital beam: In the case of estimation for analog/digital
beams, the digital beam may be estimated directly by using a
conventional orthogonal pilot allocation scheme, whereas the analog
beam requires as long a time-duration as the number of beam
candidates. This means that the more time delay is needed for
analog beam estimation, and this may cause a system loss. Moreover,
simultaneously estimating both digital and analog beams may lead to
a considerable increase in complexity.
(4) Difficulties in supporting analog beam-based spatial division
multiple access (SDMA) and FDMA: Digital beamforming allows to
freely form beams for multi-users/streams, whereas, in analog
beamforming, the same beam is formed for the entire transmission
band, making it difficult to form an independent beam per user or
per stream. In particular, it is hard to support FDMA (e.g. OFDMA)
through orthogonal frequency resource allocation, thus making the
optimization of frequency resource efficiency impractical.
Among the technical issues of the hybrid beamforming described
above, the present invention described below can provide a method
for optimizing the analog/digital beam design for the hybrid
beamforming.
Timing advance (TA) will be described. In order to maintain UL
orthogonality between receptions from different UEs in 3GPP LTE,
the timings of UL received from the different UEs are aligned in a
receiver of a BS. That is, UL transmission and DL transmission may
be aligned on the time axis at the BS. Timing alignment in UL
transmissions is one basic method for avoiding interference UEs in
a cell. TA may be applied to the UL transmission of a UE in order
to achieve timing alignment in UL transmission. A UE may set a TA
value corresponding to reception DL timing, thus dealing with
different propagation delays of different UEs.
FIG. 6 shows an example of timing alignment in UL transmission to
which no TA is applied. Referring to FIG. 6, UE1 is located
relatively close to a BS and thus has a relatively short
propagation delay TP1, while UE2 is far from the BS and thus has a
relatively long propagation delay TP2 (that is, TP1<TP2). A
criterion for the propagation delays is timing for a DL symbol that
a UE receives. Assuming that DL symbol timing in the BS is T,
timing for a DL symbol received by UE1 is T+TP1 as a result of a
delay of the propagation delay TP1 of UE1. Assuming that UE1
performs UL transmission without delay, UL symbol timing in UE1 is
T+TP1 and timing for a UL symbol received by the BS is T+2*TP1 as a
result of an additional delay of the propagation delay TP1.
Likewise, timing for a DL symbol received by UE2 is T+TP2 as a
result of a delay of the propagation delay TP2 of UE2. Assuming
that UE2 performs UL transmission without delay, UL symbol timing
in UE2 is T+TP2 and timing for a UL symbol received by the BS is
T+2*TP2 as a result of an additional delay of the propagation delay
TP2. That is, for the BS, a misalignment of 2*(TP2-TP1) occurs
between the timing of UL transmission performed by UE1 and the
timing of UL transmission performed by UE2.
FIG. 7 shows an example of timing alignment in UL transmission to
which TA is applied. By applying TA, it is possible to
appropriately achieve timing alignment in a BS. For the application
of TA, a measured propagation delay is converted into a round trip
delay (RTD), and thus a value of (propagation delay*2) is applied.
Referring to FIG. 7, assuming that the DL symbol timing in the BS
is T, timing for a DL symbol received by UE1 is T+TP1 as a result
of a delay of the propagation delay TP1 of UE1. UE1 applies a TA
value of TP1*2, and UL transmission timing in UE1 is T-TP1.
Accordingly, timing for a UL symbol received by the BS is T, and DL
transmission and UL reception are aligned in the BS. Likewise,
timing for a DL symbol received by UE2 is T+TP2 as a result of a
delay of the propagation delay TP2 of UE2. UE2 applies a TA value
of TP2*2, and UL transmission timing in UE2 is T-TP2. Accordingly,
timing for a UL symbol received by the BS is T, and DL transmission
and UL reception are aligned in the BS. It is shown that a UE that
is farther from the BS and thus has a greater propagation delay
needs to perform UL transmission relatively first for timing
alignment in the BS.
For the initial TA procedure, a UE performs initial receiver
synchronization for DL transmission from a BS and performs TA using
a random access procedure. The BS measures UL timing through a
random access preamble transmitted from the UE and transmits an
initial TA command corresponding to the UL timing through a random
access response (RAR). The TA command may have a size of 11
bits.
TA may be updated depending on the situation. The BA may perform a
TA update command using all available UL reference signals (RSs).
That is, the BS may perform the TA update command using an SRS, a
channel quality indicator (CQI), an
acknowledgment/non-acknowledgment (ACK/NACK), or the like.
Generally, a SRS may be advantageous because the accuracy of timing
estimation increases with the use of a UL RS transmitted over a
wide bandwidth. However, for a UE located on the cell boundary,
there may be a restriction on using an SRS due to power limitation.
However, since TA update depends on the implementation of the BS,
no limitation is described in the standard.
FIG. 8 shows an example of applying a TA command for TA update.
Referring to FIG. 8, the TA command for TA update is applied when a
UE transmits the first UL subframe after receiving the TA command
(5 ms-round trip time (RRT)), because a time division duplex (TDD)
frame or a half-duplex frequency division duplex (FDD) frame may
have no UL subframe at the time depending on the UL/DL
configuration. RTT may be propagation delay*2.
It has been assumed that a UE uses an omni-directional antenna
having a single channel characteristic in 3GPP LTE and thus UL
timing alignment is performed only with respect to a single channel
characteristic. However, when a UE performs narrowband beamforming
using a large number of antennas with the introduction of hybrid
beamforming, individual beams may have different scattering
environments and a timing misalignment rate may vary by each beam.
Such characteristics become more significant in a millimeter wave
(mmWave) band. In the millimeter wave band, the distribution of
scatters is changed by beamforming and path attenuation according
to the distance further increases. In the millimeter wave band,
since a hybrid beamforming structure is supported in consideration
of the ease of implementation and the complexity of a baseband, the
aforementioned beam scanning procedure may be absolutely needed. In
addition, such characteristics may become more significant when the
symbol periods of data and UL RACH are the same.
FIG. 9 is a conceptual view showing that a channel characteristic
changes by beam. As described above, in hybrid beamforming, beam
scanning needs to be performed in the time domain. However, as a
direction by beamforming is directed toward a specific scatter,
multipath characteristics may change and thus channel
characteristics may change. Referring to FIG. 9, a UE performs UL
transmission through beams #1, #2 and #3, and multipath
characteristics change by beam according to the beamforming
direction. Therefore, a BS experiences different channel
characteristics for each beam when performing beam scanning.
FIG. 10 is a conceptual diagram showing that each beam has a
different channel characteristic. Referring to FIG. 10, channel
characteristics of beam #1 and beam# 2 have different forms. In
particular, each beam has different TA timing (.tau..sub.0)
corresponding to a propagation delay.
In order to solve the foregoing problem, that is, the problem that
as channel characteristics change by beam with the introduction of
hybrid beamforming in the millimeter wave band, timing misalignment
also changes by beam, the present invention proposes a TA procedure
considering beam scanning characteristics. That is, the present
invention proposes a TA procedure for each beam considering a
change in channel characteristics by beam. According to an
embodiment of the present invention, a TA value for each beam may
be estimated considering the characteristics of an analog beam
scanning procedure for performing hybrid beamforming. That is, in
addition to an existing TA procedure based on an omni-directional
antenna, a TA procedure optimized for each beam based on
beamforming may be proposed for data transmission. In the following
description, beamforming is assumed to be analog beamforming or
hybrid beamforming, but the present invention is not necessarily
limited thereto. The present invention may also be applied to
general digital beamforming, full-dimension MIMO (FD-MIMO), and the
like.
First, according to an embodiment of the present invention, a BS
estimates a TA value for each of K different preambles, to which K
different beamformings are applied, on the time axis, or each UL RS
transmitted from one UE in UL beam scanning. That is, the BS
estimates a TA value, which changes by a change in channel
characteristics by beamforming, for each beam in UL beam scanning.
A different TA value is estimated for each beam, because a beam
scanning process is necessarily involved in view of the
characteristics of an analog terminal. The BS may estimate a UL
channel delay and a TA value for each beam through a basic
agreement about beam scanning with a UE.
FIG. 11 shows an example of a preamble transmitted by a UE for UL
beam scanning of a BS according to an embodiment of the present
invention. Referring to FIG. 11, for UL beam scanning of the BS,
the UE transmits four different preambles, to which different
beamformings are applied, to the BS. The four preambles are
transmitted via different subframes on the time axis. Further, it
is assumed that a corresponding beamforming direction is set in
each preamble.
FIG. 12 shows an example of different channel characteristics and
different total delays for respective beams according to an
embodiment of the present invention. According to the embodiment of
FIG. 11, when four different preambles are transmitted via
different beamformings, a delay profile may be different by beam
due to a different channel characteristic by beam. FIG. 12 shows
that beams have different channel characteristics are different and
thus have different delay profiles.
Table 1 shows a total delay and a TA value corresponding to each of
K beam indices.
TABLE-US-00001 TABLE 1 Beam index Total delay TA 1 .tau..sub.beam,
1 2*.tau..sub.beam, 1 2 .tau..sub.beam, 2 2*.tau..sub.beam, 2 3
.tau..sub.beam, 3 2*.tau..sub.beam, 3 . . . . . . . . . K
.tau..sub.beam, K 2*.tau..sub.beam, K
The BS estimates a channel delay and a TA value for each beam based
on K different preambles or UL RSs transmitted from the UE.
Assuming that the preambles or the UL RSs are detected in the time
domain, a channel delay m.sub.K* that maximizes output at a beam K
may be obtained by Equation 1.
.times..times..times..times..times..times..times..function..times..functi-
on..times..times. ##EQU00001##
In Equation 1, i is a time index, m is a timing offset, N is the
total length of a time signal (or OFDM symbol length), L is
multipath channel delay length by probability distribution function
(PDF), Y[i] is a signal received at time i, and S [i] is a signal
transmitted at time i. A channel delay and a TA value for each of K
different beams may be estimated through Equation 1, and
accordingly the BS may perform a TA command according to a beam
suitable for the UE.
According to an embodiment of the present invention, the BS may
perform a TA command to the UE based on a channel delay or a TA
value estimated for each beam. In detail, the BS may perform a TA
command in view of the channel delay or the TA value of at least
one of the K different beams detected by beam scanning. The BS may
determine the final TA value of the UE based on various criteria.
The BS may determine the final TA value of the UE based on one
beam. For example, the BS may determine the final TA value of the
UE based on the TA value of a beam having the best reception
quality among the detected beams. Alternatively, the BS may
determine the final TA value of the UE based on a plurality of
beams. For example, the BS may determine the final TA value of the
UE based on the TA values of a plurality of beams having a signal
strength or signal quality of a threshold or higher among the
detected beams. Hereinafter, various embodiments in which the BS
determines or selects the final TA value of the UE will be
described.
(1) Determine a final TA value corresponding to a selected beam:
The BS may select a beam having the best reception quality among
detected beams and may determine a TA value corresponding to the
selected beam as a final TA value. That is, the final TA value may
be determined by Equation 2. Total TA=2.times..tau..sub.beam, K
[Equation 2]
For example, when beam #2 has the best reception quality among K
beams, a final TA value may be determined to be
2*.tau..sub.beam,2.
(2) Determine the average value of a TA value for conventional
synchronization and a TA value for a selected beam as a final TA
value: The BS may determine, as a final TA value, the average value
of a TA value 2*.tau..sub.TA used for conventional UL
synchronization and a TA value 2*.tau..sub.beam,best for a beam
having the best reception quality among detected beams. That is,
the final TA value may be determined by Equation 3.
.times..times..times..tau..times..tau..times..times.
##EQU00002##
(3) Determine the average value of a TA value for a beam having the
best reception quality and a TA value for a beam having the second
best reception quality as a final TA value: The BS may determine,
as a final TA value, the average value of a TA value
2*.tau..sub.beam,best for a beam having the best reception quality
and a TA value 2*.tau..sub.beam,2nd for a beam having the second
best reception quality. That is, the final TA value may be
determined by Equation 4.
.times..times..times..tau..times..tau..times..times..times.
##EQU00003##
FIG. 13 shows an example of determining a final TA value based on a
TA value estimated for each beam according to an embodiment of the
present invention. In FIG. 13, assuming that beams #1 and #2 are
respectively selected as a beam with the best reception quality and
a beam with the second best reception quality among K different
beams, the average value of a TA value for beam #1 and a TA value
for beam #2 is determined as a final TA value.
(4) Determine the average value of the TA values of all beams
having a reception strength of a specified threshold or higher as a
final TA value: The BS may selects all beams having a reception
strength of a specified reference power P.sub.threshold or higher
and may determine the average value of the TA values of the beams
as a final TA value. That is, the final TA value may be determined
by Equation 5.
.times..tau..times..tau..times..times..tau..times..times..times..times..t-
imes..times..times..times..times.>.times..times.
##EQU00004##
In Equation 5, KS represents the total number of selected beams. In
addition, an average TA may be derived by applying a weight
according to received signal power.
FIG. 14 shows a method for a BS to estimate a TA for each beam
according to an embodiment of the present invention.
In step S100, the BS receives K preambles, to which K different
beamformings are applied, from a UE. The K preambles may be
received through different subframes. Alternatively, the K
preambles may be successively received in the same subframe. The
beamformings may include at least one of analog beamforming,
digital beamforming, and hybrid beamforming.
In step S110, the BS estimates a TA for each of the K preambles.
Estimating a TA for each of the K preambles may include estimating
a channel delay for each of the K preambles, in which Equation 1
may be used to estimate a channel delay for each preamble.
The BS may determine the final TA value of the UE based on the TA
estimated for each of the K preambles. The final TA value of the UE
may be determined based on a TA value corresponding to one preamble
selected from the K preambles. For example, the final TA value of
the UE may be a TA value corresponding to a preamble having the
best reception quality among the K preambles. The final TA value of
the UE may be the average value of a TA value used for conventional
synchronization and a TA value corresponding to a preamble having
the best reception quality among the K preambles. In another
example, the final TA value of the UE may be determined based on TA
values corresponding to two or more preambles among the K
preambles. The final TA value of the UE may be the average value of
a TA value corresponding to a preamble having the best reception
quality among the K preambles and a TA value corresponding to a
preamble having the second best reception quality. The final TA
value of the UE may be the average value of TA values corresponding
to preambles having a reception quality of a specified threshold or
higher among the K preambles.
The BS may transmit a TA command including the determined final TA
value of the UE to the UE.
FIG. 15 shows a method for a UE to apply a TA according to an
embodiment of the present invention.
In step S200, the UE transmits K preambles, to which K different
beamformings are applied, to a BS. The K preambles may be
transmitted through different subframes. Alternatively, the K
preambles may be successively transmitted in the same subframe. The
beamformings may include at least one of analog beamforming,
digital beamforming, and hybrid beamforming.
In step S210, the UE receives a TA command including a final TA
value, which is determined by the BS based on the K preambles, from
the BS and applies the TA. The BS may determine the final TA value
based on the K preambles using various methods described in the
present specification.
FIG. 16 shows a TA procedure according to an embodiment of the
present invention.
In step S300, the UE transmits K preambles, to which K different
beamformings are applied, to a BS. The K preambles may be
transmitted through different subframes. Alternatively, the K
preambles may be successively transmitted in the same subframe. The
beamformings may include at least one of analog beamforming,
digital beamforming, and hybrid beamforming.
In step S310, the BS estimates a TA for each of the K preambles.
Estimating a TA for each of the K preambles may include estimating
a channel delay for each of the K preambles, in which Equation 1
may be used to estimate a channel delay for each preamble.
In step S320, the BS determines a final TA value based on a TA
estimated for each of the K preambles. The BS may determine the
final TA value using various methods described in the present
specification.
In step S330, the BS transmits a TA command including the final TA
value to the UE.
The above description of the present invention shows that a
plurality of beams is transmitted from a single antenna array, this
is merely an example. The present invention described above may
also be applied to the transmission of one beam from each of some
antennas arrays. In this case, each beam may be applied as a
transmission beam of each sub-array.
Further, the foregoing present invention may be applied to a
combination of a transmitter and a receiver. If the transmitter is
a BS and the receiver is a UE, the foregoing present invention may
be applied to DL. If the transmitter is a UE and the receiver is a
BS, the foregoing present invention may be applied to UL. If both
the transmitter and the receiver are UEs, the foregoing present
invention may be applied to a side link.
The foregoing present invention may be applied to beamforming or
precoding through both analog and digital processing with respect
to a multi-antenna. When the foregoing present invention is applied
to a broadband system, a broadband may be divided into specified
frequency domains (for example, sub-bands, subcarriers, resource
blocks, or the like), and a separate feedback information set may
be transmitted with respect to each frequency domain.
Alternatively, feedback information may be transmitted only with
respect to a particular frequency domain selected by a UE or
designated by a BS. The frequency domain may include one or more
successive areas on the frequency axis or may include one or more
non-consecutive areas on the frequency axis.
FIG. 17 shows a wireless communication system to implement an
embodiment of the present invention.
A BS 800 includes a processor 810, a memory 820 and a transceiver
830. The processor 810 may be configured to implement proposed
functions, procedures and/or methods described in this description.
Layers of the radio interface protocol may be implemented in the
processor 810. The memory 820 is operatively coupled with the
processor 810 and stores a variety of information to operate the
processor 810. The transceiver 830 is operatively coupled with the
processor 810, and transmits and/or receives a radio signal.
A UE 900 includes a processor 910, a memory 920 and a transceiver
930. The processor 910 may be configured to implement proposed
functions, procedures and/or methods described in this description.
Layers of the radio interface protocol may be implemented in the
processor 910. The memory 920 is operatively coupled with the
processor 910 and stores a variety of information to operate the
processor 910. The transceiver 930 is operatively coupled with the
processor 910, and transmits and/or receives a radio signal.
The processors 810, 910 may include application-specific integrated
circuit (ASIC), other chipset, logic circuit and/or data processing
device. The memories 820, 920 may include read-only memory (ROM),
random access memory (RAM), flash memory, memory card, storage
medium and/or other storage device. The transceivers 830, 930 may
include baseband circuitry to process radio frequency signals. When
the embodiments are implemented in software, the techniques
described herein can be implemented with modules (e.g., procedures,
functions, and so on) that perform the functions described herein.
The modules can be stored in memories 820, 920 and executed by
processors 810, 910. The memories 820, 920 can be implemented
within the processors 810, 910 or external to the processors 810,
910 in which case those can be communicatively coupled to the
processors 810, 910 via various means as is known in the art.
In view of the exemplary systems described herein, methodologies
that may be implemented in accordance with the disclosed subject
matter have been described with reference to several flow diagrams.
While for purposed of simplicity, the methodologies are shown and
described as a series of steps or blocks, it is to be understood
and appreciated that the claimed subject matter is not limited by
the order of the steps or blocks, as some steps may occur in
different orders or concurrently with other steps from what is
depicted and described herein. Moreover, one skilled in the art
would understand that the steps illustrated in the flow diagram are
not exclusive and other steps may be included or one or more of the
steps in the example flow diagram may be deleted without affecting
the scope and spirit of the present disclosure.
* * * * *